With the development of industrial society, various types of pollutants have been increasing in water from streams, lakes, and underground. Thus, as an amount of water resources of good quality reduces, drinking water production becomes more difficult, and available water resources reduces, the need to re-use treated wastewater dramatically increased.
Particularly, in recent years, toxic trace elements such as heavy metals, pharmaceuticals and personal care products (PPCPs), and endocrine disrupting chemicals (EDCs) increase in water from streams, lakes and underground, and effluent in treated wastewater, which threatens the production of drinking water of good quality and water re-use. Because toxicity of toxic trace elements is high even at a low concentration, toxic trace elements must be removed in a water purification process and a water recycling process. Typical examples of heavy metals include lead, cadmium, and chrome, typical examples of pharmaceuticals and personal care products include ibuprofen, acetaminophen, oxytetracycline, and caffeine, and typical examples of endocrine disrupting chemicals include 17α-ethinylestradiol (EE2), 17β-estradiol (E2), estrone (E1), and bisphenol A (BPA).
As opposed to other pollutants, toxic trace elements are not removed in general water purification and sewage and wastewater treatment processes. To remove toxic trace elements, coagulation, adsorption, biological treatment, ion exchange, membranes, and advanced oxidation processes (AOPs) are being attempted. However, they have problems such as low efficiency, high costs and secondary pollution, which limit the commercialization. Particularly, activated carbon is primarily used in a water purification process and a sewage and wastewater treatment process, and activated carbon has good adsorption performance of organic pollutants, but when organic pollutants more than a maximum adsorption capacity are adsorbed (saturated), saturated activated carbon cannot adsorb and remove an amount of pollutants any more. The saturated activated carbon needs to be replaced with new activated carbon. It is possible to reactivate saturated activated carbon, but a large amount of energy and costs is required for reactivation, secondary pollutants are created, and a partial loss occurs during reactivation. Also, activated carbon is known as having low adsorption performance of toxic trace elements.
Recently, to remove toxic trace elements, metal oxide nanoparticles are gaining attention. Metal oxide nanoparticles generally refer to particles having an individual particle size of 100 nm or less, and iron oxide, iron oxide and iron hydroxide, titanium oxide, and manganese oxide particles are mainly being studied. In the removal of pollutants in the water, attempts are being made to use metal oxide nanoparticles in a variety of applications, for example, catalysts, adsorbents, and ion exchange materials, and in particular, more recently, attention is being paid to metal oxide nanoparticles as heterogeneous catalysts or adsorbents for organic pollutants oxidation/reduction.
Advantages of metal oxide nanoparticles are as follows. Firstly, a specific surface area is large, reactive sites and adsorption sites are rich on the surface, and organics oxidative removal performance and heavy metals adsorptive removal performance is very high. For example, research has reported that iron oxide nanoparticles have a phenol removal rate about 35 times higher than and an ethylene glycol removal rate 2 times to 4 times higher than the Fenton oxidation process traditionally used to remove non-biodegradable organics (Zelmanov and Semiat, 2008). Also, it was reported that micro-sized zinc oxide is incapable of adsorb and remove arsenic (As), but nano-sized zinc oxide has good arsenic adsorption and removal performance (Tiwari et al., 2008). Secondly, because pollutant removal efficiency per unit weight is high due to a large specific surface area, when removing the same amount of pollutants, an amount of injection is lower than particles having a larger particle size, so it may be used with economic efficiency.
However, metal oxide nanoparticles are difficult to commercialize due to the following drawbacks. Firstly, it takes a large amount of energy and toxic chemicals such as acid or alkali, an oxidant or a reducing agent, and a dispersant to prepare metal oxide nanoparticles, so there is a high likelihood that environment pollution and a safety-related problem will occur in the preparation process. Secondly, because nanoparticles have a small size less than or equal to 100 nm, they are difficult to separate by precipitation and filtration. That is, when nanoparticles are fed into a reactor for water treatment, they are released into water bodies together with treated water after treatment of pollutants and contaminate water from streams, lakes, and underground.